In a typical wireless architecture a transmitting source is required to be kept at a relatively constant average power level over a period of transmission time. Because of temperature variations of individual gain stages along the transmit path (caused by higher current draw when transmitting) the output power tends to droop over time. The environmental temperature in which the transmitting device operates in can also cause the gain to fluctuate. In order to account for this difference in gain over temperature, a Radio Frequency (RF) power detection circuit is typically placed after the final gain stage of the transmit path in order to create a feedback signal which will adjust the gain in the beginning of the transmit path in order to account for the transmit path's droop or rise in gain. The detection typically consists of a power coupler, which couples a small portion of the electromagnetic waveform from the output of the last gain stage, and a properly biased diode circuit. The diode output is an analog voltage, which is an exponential function of the input power, in dBm. This signal is then scaled and fed into an Analog/Digital Converter (ADC), which converts the signal to a digital level used to adjust the power into the transmit path accordingly.
The problem with this method is that the analog signal created by the diode has a “sweet spot” due to the exponential behaviour of its response to RF energy. A sweet spot occurs when there are many ADC counts for a small range of power. At lower powers (power being referenced in dB) the voltage/dB output of the diode is lower than at higher powers. For example, for a diode at low powers, it may be necessary to raise the power 3 dB to get a 0.01V change, where at the higher powers, raising the power 3 dB will vary the voltage by 0.5V. At the ADC, this results in less voltage resolution and thus more A/D steps per dB, and causes inaccuracies in power feedback at the lower power levels. Typically the analog voltage from the diode is scaled, possibly by an Operational amplifier (Opamp) to use the full scale of the A/D window of the detector. The gain of the Opamp can properly scale the analog voltage, but the lower powers will still be represented with a lower resolution.
The typical method for resolving this problem was to set the sweet spot of the diode to centre at the higher powers that the device would detect and then live with less accurate output power at the lower powers. Another option is to not use the power detector at lower powers, assuming the parts will not heat up as much. Either option is subject to less accurate output power at lower levels over a wide range.
In summary, the problem is that there exists no practical solution to represent the analog exponential voltage created at the diode over a large range of power, e.g. greater than 10 dB to 15 dB, and retain bit resolution needed by the ADC to accurately report power. The present invention addresses this problem.